Field of the Invention
The present invention relates to a measuring device having a two-wire field device, a two-wire field device, as well as a method for operating a two-wire field device.
Background of the Invention
In process automation technology frequently field devices are used, which detect and/or influence process variables. Examples for such field devices are level gauges, limit meters, and manometers having sensors, which detect the respective process variables fill level, limit, or pressure. Frequently such field devices are connected to superordinate units, for example guidance systems or control units. These superordinate units are used for process control, process visualization, and/or process monitoring.
The energy transmission and/or signal transmission between the field device and any superordinate units occurs frequently according to the known 4 mA to 20 mA standard, in which a 4 mA to 20 mA current loop and/or a two-wire line is formed between the field device and the superordinate unit. In addition to the analog transmission of signals, there is the option that the measuring devices as per various other protocols, particularly digital protocols, transmit additional information to the superordinate units or receive them therefrom. Examples to be mentioned here are the HART-protocol or the Profibus-PA protocol.
The energy supply of these field devices also occurs via the 4 mA to 20 mA current signal, and therefore in addition to the two-wire line no additional supply line is required. In order to keep as low as possible the wiring expense and installation expense as well as the safety measures, for example when used in areas to be protected from explosion, it is not desired to provide additional current supply lines.
From the prior art for example control units are known which are connected to the 4 mA to 20 mA current loop for physically communicating with the field device, in order to in this manner allow the operation of the field device via the 4 mA to 20 mA current loop by means of the HART-protocol. Such control units are disadvantageous in that, as already mentioned, they must be physically connected to the 4 mA to 20 mA current loop. For this purpose, the two-wire line must be modified in such a manner, for example isolated and/or severed, that the control unit can be connected thereto. This is relatively labor-intensive and is not always readily possible on-site. Alternatively a control device can also be connected to an existing HART-resistor, and in that case no severing of the line is necessary.
It is further known from the prior art to use radio modules for operating the field devices in an easier manner. Here, solutions are known in which the field devices comprise integrated radio modules, radio modules are retrofitted, or they are arranged in a measuring transformer—feeding device, a device for supplying energy to the field devices. In particular in case of the variants having an integrated radio module or having a radio module that shall be retrofitted for example with modularly designed field devices, the problem exists in the prior art that an energy supply via the 4 mA to 20 mA current loop allows only extremely low transmission powers of the radio modules or the operation of the radio module is only possible with low transmission power due to the energy available being limited, or the energy supplied via the current loop is insufficient to supply the radio module with sufficient energy, and therefore operation is impossible or is only possible with considerable limitations.
Here, solutions are known from the prior art in which the radio module is provided with a separate energy supply or additional energy sources are provided, such as batteries, energy harvesters, or the like for operating the radio module in the field device. Both of the above-mentioned solutions have an increased installation requirements and maintenance requirements, and therefore they are considered disadvantageous.
The problem addressed by the present invention is that of providing a measuring device having a two-wire field device, a two-wire field device, as well as a method for operating a two-wire field device which solves the disadvantages of the prior art and particularly can forgo any additional energy supply, particularly additionally wired or a partially or completely battery operated energy supply.
Said problem is solved by a measuring device, a two-wire field device, as well as a method for operating a two-wire field device, each having the features described herein.